Is that 0.050 M acetic acid? Let HAc stand for CH3COOH.
You want pH = 3.80, convert that to (H^+) by pH = -log(H^+).
Ka = (H^+)(Ac^-)/(HAc)
Substitute (H^+),(Ac^-), and Ka into the Ka expression and solve for (HAc). Then
substitute into the dilution equation to finish.
m1C1 = m2C2